Such a pulley is for example proposed in document EP 0,980,479 (D1).
This pulley is shown in FIG. 1, in a longitudinal sectional view.
The pulley P comprises a wheel rim J and a hub M that can be coupled to one another via a torsion spring RT and a unidirectional clutch EU mounted in series with the torsion spring RT.
The torsion spring RT is built and arranged to transmit rotational movements imparted to the wheel rim J, for example by a belt mounted on the wheel rim and connected to a drive shaft, to the hub M such that the hub M, which is for example intended to be mounted on a shaft of an auxiliary device such as an alternator, can be driven in the same direction as the wheel rim (“coupling” mode).
The unidirectional clutch EU is for example built and arranged such that the hub M, and from there, the shaft on which this hub is intended to be mounted, can rotate at a speed exceeding the rotation speed of the wheel rim J, in particular when the pulley is decelerated, for example due to a deceleration of the motor (“overrunning mode”).
Thus and traditionally, in “coupling” mode, the motor torque passes through the belt, the wheel rim J, the unidirectional clutch EU, which is then driven by friction, relative to the wheel rim J, the torsion spring RT, which is connected in series with the unidirectional clutch EU, the hub M, on which the torsion spring RT is mounted, and which then comes into contact with a component C fastened to the hub M, and lastly, the shaft on which the hub is mounted.
On the contrary, in the “overrunning mode”, the unidirectional clutch EU disengages from the wheel rim J and the torsion spring RT, in series with the unidirectional clutch in the neutral position with zero torque.
One drawback of the pulley proposed in document D1 lies in the fact that the torsion spring RT may undergo very significant deformations (radial expansions) in the “coupling” mode. Indeed, in this operating mode, the torque passes through the torsion spring RT, without the radial expansion of the torsion spring, resulting from the application of the torque, being limited.
Consequently, the radial expansion of the torsion spring RT may be such that the latter comes into contact with the unidirectional clutch EU. Indeed, in document D1, the unidirectional clutch EU, which is placed below the receiving zone DE for the belt, is placed directly around the torsion spring RT.
This may make the device nonoperational.
This problem is known, and solutions to limit the radial expansion of the torsion spring have already been proposed.
Thus, to avoid this problem, document U.S. Pat. No. 7,975,821 B2 (D2) proposes implementing an intermediate part (referenced 110 in FIG. 2 of document D2) between the torsion spring and the unidirectional clutch.
The solution proposed in document D2, however, involves a pulley diameter at the receiving zone for the belt larger than the diameter of the pulley proposed in document D1.
This may pose practical difficulties, since the maximum acceptable diameter of the pulley at this belt receiving zone, defined by the machine or vehicle, in particular motor vehicle, builders, does not allow it to be used in any type of application. This limits the gear reduction ratio obtained between the crankshaft of the engine and the alternator. Practically speaking, an effective diameter of the toothing zone of 50 mm is impossible, but desirable. As a result, this also limits the possibilities for the sizing of the spring forming the unidirectional clutch.
Yet the torque transmitted between the wheel rim and the hub also passes through the unidirectional clutch, which further reduces the sizing possibilities for the unidirectional clutch.